The present invention relates generally to the field of moisture detectors, and, in particular, to a novel mechanism and method of signal processing for sensing of moisture in building construction materials, including wood, concrete, gypsum, roofing felt, various flooring and other materials.
Although the approach illustrated is intended for use in moisture sensing by surface contact with a measurement sample, this is the preferred embodiment for the invention, with variations possible in sensor shape and configuration, and electronic circuitry described herein, known to those skilled in the art to be considered as integral to the spirit and scope of this disclosure.
It is an object of this disclosure to illustrate novelty of the design approach and reading stability offered by the same.
Many approaches to capacitive moisture measurement have been utilized over a substantial number of years.
Virtually all rely upon creation of an electrostatic field within the sample undergoing moisture measurement, where the alternating current potential component between two electrodes within the field is measured. Such an arrangement is described in U.S. Pat. No. 3,967,197.
Alternatively, some designs utilize a three electrode configuration, where a form of voltage division occurs between a driven and common or ground electrode, and a receiving electrode via the induced electrostatic field. Often this alternative geometry is implemented in the form of a center receiving electrode, surrounded by a co-planar driven or transmitting ring, in turn surrounded by a coplanar ground plane surface. Such an arrangement is described in U.S. Pat. No. 5,486,815, which specifically utilizes a rectangular arrangement of the stated three electrode implementation. Other approaches use a multiplicity of electrodes for specific measurement purposes.
Electronic circuitry for sensor excitation and reading of sensor output generally consists of an oscillator for excitation, and a diode detector for development of a DC voltage related to peak, average, or other parameter which relates the AC component to a DC value required for display purposes, be the display analog or digital. Commonly the diode detector is temperature compensated to an extent in order to minimize accuracy degradation due to the well known phenomenon of diode voltage drop dependence upon temperature. In silicon diodes, this effect is approximately −2.2 millivolts per degree Celsius. In virtually all instances, another diode or PN junction of a bipolar transistor is used for compensation, but due to circuit design constraints, it cannot be employed in such a way that the current versus time profile is exactly the same as the measurement rectification diode.
While the subject profile in each of the diodes or other PN junction devices can be quite similar, the profiles cannot be exactly the same, or no way will exist for extraction of a DC signal from the receiving electrode of the sensor. Dissimilarities between diode current profiles, while small, become meaningful due to the need for implementing temperature compensation involving very small signals. In some instances of instrument operation, there are differences of only several millivolts in sensor signal output when sensing moisture content of certain relatively dry materials. Under such conditions, temperature of the measurement instrument may meaningfully influence moisture reading accuracy. Indeed, some design approaches totally ignore the degrading effects of diode temperature. One example is found in U.S. Pat. No. 4,733,166, where a diode 82 in FIG. 4 thereof, serves as an RF amplitude detector, but is not voltage drop or temperature compensated.
A need remains for an improved moisture detector, particularly one with improved temperature independence and improved moisture signal sensitivity.